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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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HIDDEN, FOUETH AND SMALLER GLACIERS                  157
involved, there can be no doubt but that tM-g advance also was spasmodic and in every important respect similar in character to the advance of the other glaciers, the one notable difference being the much greater advance of the front; for in this respect Hidden Glacier stands unique among all the advancing glaciers. While in all cases there was some advance of the front, and in the Haenke Glacier a notable forward movement, there was nothing to compare with the remarkable change in position of the Hidden Glacier front. Doubtless could the phenomenon of advance have been observed it would have presented 'Only a duplication of the same features shown by the other glaciers; but it would have been most interesting to have observed how rapidly the front of the glacier actually moved forward, and also to have seen what response to the thrust the ice buried beneath the outwash gravel plain made. Upon these questions, however, we cannot now hope to tave evidence.
As to the exact time of the advance of the glacier we are also unable to make positive .statement. It is known to have come between July 10, 1906, and July 6, 1909. There are numerous indications that the advance came nearer the earlier period than the later, and it may even have come later in the season of 1906 than the period of our visit. We cannot conceive that it occurred later than the summer of 1907, for after the advance ,and breaking were completed, and before 1909, there had been enough ablation to reduce the crevassed, broken surface of the advancing glacier to a passable condition. The full season of 1908, and the first month or two of 1909, certainly represent none too much time for as much healing of the surface by ablation as has taken place. The surface •was rougher in 1909 than that of the Variegated Glacier, whose breaking had been so nearly completed in the summer of 1906 as to give ablation some play in that season while during 1907 and 1908 ablation doubtless dominated in the Variegated Glacier. In view of the greater roughness of the surface of Hidden Glacier we are, therefore, inclined to place its period of advance after the close of the summer season of 1906 and very probably before the period of melting in 1907 had proceeded very far.
In addition to the healing of the surface by ablatidh, the marginal conditions along the northern side of the glacier are suggestive of the lapse of a period of at least one or two ^years since the maximum advance. This evidence is partly the notable shrinking of -the ice from its most advanced position on the northern slopes, and partly the rock gorge 'Occupied by the marginal channel, The bushes killed by the advance were so brittle in 1909 that limbs snapped readily, suggesting that they had been overturned as early as 1906 or 1907. The condition of the glacier front was also indicative of the lapse of at least one full season since the advance, for, in order to develop the cliffed front of the .glacier and the fosse, and in order to lower the frontal slopes sufficiently for the deposit of the alluvial fan described above, would seem to demand at least one complete season -of ablation. Altogether, therefore, although we cannot state with definite exactness the time at which the Hidden Glacier advanced, we believe that the evidence from ablation and from frontal and marginal conditions demands that this period should be at least before the beginning of the summer of 1908, and later than the autumn of 1906,— that is, some time in 1907. That the advance was rapid and soon ended is proved by the great change in position and condition, for a very large part of the three years available is Tequired for the observed healing by ablation. In other words, the evidence from Hidden 'Glacier, though in some respects lacking definiteness, is completely in harmony with that burnished by the other advancing glaciers, about which we have more exact information.